This invention relates to a hollow fiber membrane module to be used in industrial fields such as the semiconductor industry, food processing industry, pharmaceutical industry, and medical industry and to a method for the production thereof. More particularly, this invention relates to a hollow fiber membrane module which is suitable for a use requiring a smaller decline in flow volume of a fluid between the initial stage and the latter stage of filtration.
Generally, a hollow fiber membrane module is formed by causing a hollow fiber membrane wound on a bobbin to be rewound on a rewinding jig having a polygonal cross section. One side of a coil formed of a multiplicity of hollow fiber membranes rewound on the jig is then cut, thereby giving rise to a membrane bundle. The openings in the opposite terminal parts of the membrane bundle are stoppered in a straight form or in a looped form lest the interiors of the membranes of the bundle be invaded by a potting resin. Thereafter, the stoppered portions are potted, thereby completing sealed parts (as disclosed in JP-B-07-106,302, for example).
As another hollow fiber membrane module, the construction of a membrane bundle obtained by using hollow fiber membranes as wefts, knitting the wefts across warps after the fashion of a textile to produce a sheet, and rolling the sheet of fabric around itself has been proposed. The construction of a membrane bundle obtained by rolling the aforementioned sheet of fabric around itself with a mesh interposed between the superposed plies of the roll has also been known (as disclosed in JP-A-62-57, 965, for example).
These hollow fiber yarn bundles open in at least one terminal part of membrane bundle, and a diaphragm adapted to prevent the fluids flowing inside and outside the hollow fiber membranes from mingling with each other is disposed in the opening. This diaphragm uses an adhesive agent of thermosetting resin such as epoxy, urethane, or silicone rubber. These thermosetting resins have low initial viscosity, are suitable for the purpose of immersion-solidifying or injection-solidifying a permselective membrane or a microporous membrane which has small rigidity, and are particularly optimum for a membranous material of the shape of a hollow fiber.
When such a thermosetting resin is utilized for the technique of potting hollow fiber membranes, a large number of hollow fiber membranes are uniformly dispersed at the potting position, and the thermosetting resin fills the gaps between the hollow fiber membranes. If the thermosetting resin increases in viscosity, it will no longer be able to fill the gaps between the hollow fiber membranes or produce a fully satisfactory sealed state.
As the potting resin for filling the gaps between the hollow fiber membranes, therefore, a thermosetting resin which has low viscosity is used in particular. Such is the true state of the prior art.
When the hollow fiber membrane bundle having the straight shape or looped shape mentioned above is subjected to vertical filtration, however, the efficiency of filtration gradually degrades by clogging. It is known that the degree of the consequent degradation of flow volume, when rated is based on a fixed membrane area, decreases in proportion as the number of membranes decreases and the length of membrane increases.
In this case, the increase in membrane length requires the length of the module to be increased and, as a result, entails a disadvantage of making incorporation of modules in a given device difficult. An attempt to have long hollow fibers accommodated in a short module case causes breakage of fibers and inevitably adds to the number of hours of work for accommodation. Since the conventional hollow fiber membranes individually allow a large degree of freedom and easily induce relevant fluids to drift, they make it difficult for materials in a fluid to transfer through the membrane, and prevent the membrane from effectively achieving filtration efficiency.
The specification of a plain coil of the sheet of textile mentioned above embraces a device for preventing the drift by the intervention of a mesh. However, since this device adds to the number of component members, results in increased costs, and enhances the chance of yielding an extracted matter from the materials themselves which are required by the module in the existing state, the number of component members of the module is preferred to be as small as permissible.
As the potting resin for the hollow fiber membrane, a thermosetting resin having low viscosity is used as mentioned above. In the semiconductor industry, the materials for semiconductors have been in need of chemical resistance, with the gradual rise in the degree of integration of semiconductors as a contributory factor. Thus, the module relevant herein ought to adopt, as the potting material, a thermoplastic resin which is identical or similar to the thermoplastic resin for a hollow fiber membrane. Incidentally, the thermoplastic resin generally has a high melting temperature and exhibits high viscosity at that temperature and burdens the potting technique with various problems yet to be solved.
A thermoplastic resin favors adoption of the immersion or injection potting because it has high viscosity and consequently makes adoption of the centrifugal potting difficult. In this case, since hollow fiber membranes themselves have a small amount of rigidity and inevitably yield to the pressure of insertion into the thermoplastic resin or the pressure of injection of the resin, they are partly distributed unevenly and are unevenly dispersed in the terminal parts of membranes. Since this unevenness prevents sufficient filling of the gaps between the hollow fiber membranes with the potting resin, the membranes allow communication between the primary side and the secondary side to the extent of inevitably impairing the filtering function thereof.
When the method of crushing (for example, by thermosetting resin, thermoplastic resin, thermal deposition, or shearing) is carried out for stoppering membranes in advance of the adoption of the immersion or injection potting technique, this method results in degrading the operational efficiency because the hollow fiber membranes which have undergone the stoppering treatment no longer allow effective potting unless they are separated one by one.
The problem further arises that the terminal parts of the hollow fiber membranes generate voids which are bubbles of vacuum, unless the thermoplastic resin is deprived of strain by shrinkage before it is allowed to set. The hollow fiber membranes generally are subjected to centrifugal cooling after they have undergone the potting treatment. This cooling, however, brings the problem of adding to the number of component steps of the process.
This invention has been perfected in view of a diligent study initiated as a result of the true state of prior art described above. It has for an object thereof the provision of a compact hollow fiber membrane module, which precludes the occurrence of defective potting by failing to completely fill the gaps between the hollow fiber membrane with a resin, obviates the necessity for a stoppering step, further obviates the necessity for a step of removing voids occurring during the course of solidification, avoids the possibility of inducing fluids inside and outside the hollow fiber membrane to drift, effects material transfer in fluids through a membrane with high efficiency, and allows accommodation of long hollow fiber membranes in a short module. It is particularly directed at providing a hollow fiber membrane module which is endowed with enhanced efficiency of filtration and consequently adapted for use in the field of semiconductor industry calling for resistance to chemicals, for example.
To accomplish the object mentioned above, this invention provides a hollow fiber membrane module which is obtained by cheese-winding a plurality of layers of permselective hollow fiber membranes made of thermoplastic resin, thereby forming a rigid membrane bundle. At least in one terminal part of the membrane bundle, a sealed part of thermoplastic resin serves to open the terminal faces of the hollow fiber membranes. The module can be applied to any one of a deaeration module, a membrane type drier, pervaporation (a method of vaporation by permeation) and a bioreactor.
Further, the present invention contemplates cheese-winding a plurality of layers of permselective hollow fiber membranes made of thermoplastic resin, thereby forming a rigid hollow fiber membrane bundle. In at least one terminal part of the hollow fiber membrane bundle, a sealed part of thermoplastic resin serves to open the terminal faces of the hollow fiber membranes together with a case in a sealed state, and the case encloses the outer periphery of the hollow fiber membrane bundle in a tight state. The resultant module is applied to any one of a deaeration module, a membrane type drier, pervaporation (a method of vaporation by permeation) and a bioreactor.
In this case, the adjoining hollow fiber membranes are superposed in a plurality of layers as cheese-wound in a self-supportably tight state, thereby forming a membrane bundle.
When a core tube is removed from the cheese-wound membrane bundle, a hollow part is formed in the portion formerly occupied by the core tube. The efficiency of filtration of the module is enhanced by inserting a spacer in that hollow part to preclude the occurrence of a dead space for the fluids being treated.
Further, the hollow fiber membranes are formed of a thermoplastic resin, and the sealed part is formed of a thermoplastic resin. Particularly, the thermoplastic resin is polyolefin such as polyethylene or polypropylene, fluorine resin, polyamide or polyimide, for example.
The hollow fiber membrane bundle is intended for allowing a fluid under treatment to be internally refluxed by producing a fluid motion inside the hollow fiber membranes or for allowing the fluid under treatment to be externally refluxed by producing a fluid motion outside the hollow fiber membranes.
The method of this invention for the production of a hollow fiber membrane module comprises cheese-winding a plurality of layers of hollow fiber membranes of thermoplastic resin permselective to a fluid on a core tube at a prescribed angle of winding, thereby forming a membrane bundle. At least one terminal part of the membrane bundle is adhered to a thermally molten mass of thermoplastic resin identical or similar to the thermoplastic resin of the hollow fiber membranes, thereby forming a solidified part. The solidified part is then cut to open the terminal parts of the hollow fiber membranes, thereby forming a sealed part.
In this case, the sealed part is obtained by providing a resin reservoir at a position underlying a resin layer placed in a die, and immersing in the resin layer one terminal part of the membrane bundle together with the case enclosing the outer periphery of the hollow fiber membrane bundle in a tight state, thereby causing adhesion and solidification of the one terminal part with the resin so as to form a solidified part. The solidified part is ten cut so as to open the terminal parts of the hollow fiber membranes. The ingot piping which is formed in the reservoir of resin is cut and removed by separating the portion of the solidified part which sets the resin reservoir.
In this case, the angle of cheese-winding, xcex8, is in the range of 0xc2x0 less than xcex8 less than 90xc2x0.
Since the hollow fiber membrane module contemplated by this invention is constructed as described above, the length of the hollow fiber membranes can be increased by properly selecting the angle of cheese-winding. Consequently, the combined capacity of the hollow fiber membranes per unit volume can be increased as compared with the conventional modules. Further, since the hollow fiber membrane module provided by this invention has long hollow fiber membranes accommodated in a short module, the invention permits production of a module so compact and so capable of effective filtration as to be infallibly mounted in a pipeline having a small interfacial dimension.
Further, the hollow fiber membrane module enjoys conspicuous improvement in yield because the membrane bundle continues to retain rigidity, attains immersion into or injection with the molten resin having great viscosity, and permits uniform dispersion of resin without yielding to the pressure of insertion into or the pressure of injection with the resin.
Further, the process for production according to the present invention obviates the necessity for stoppering hollow fiber membranes. This fact allows a decrease in the number of component steps of the process, improves the operational efficiency, and contributes to the reduction in cost. Moreover, the speed of potting can be increased because the potting step is allowed to utilize the resin reservoir, and the subsequent step is adapted to cut and separate the resin reservoir having the ingot piping when the solidified part is cut. This point also encourages an attempt to improve the yield and the operational efficiency of potting.
Since the hollow fiber membranes and the sealed part use the thermoplastic resin, the liquation of organic substances and metal ions can be alleviated, and the adverse effects of organic solvents and various chemicals can be coped with.
Further, the hollow fiber membrane module according to this invention has a conspicuous effect on the operation of filtration due to the material transfer implemented by the use of a deaeration module, a membrane type drier, pervaporation (a method of vaporation by permeation), and a bioreactor because it allows no easy occurrence of the phenomenon of channeling, offers stable performance, and brings no dispersion of quality in product. Furthermore, since a hollow fiber membrane module having long hollow fiber membranes can be provided even when mounted in a predetermined interfacial dimension, it enables the material transfer to proceed efficiently in the hollow fiber membranes, and notably improves the performance owing to an elongation of the duration of contact. It also contributes greatly to increase the power of filtration and has a fine effect in the semiconductor industry calling for resistance to chemicals, for example, because it can cope stably with the dynamical pressure of the fluid being filtered.